Future work using natural lignin and more representative solution compositions will be needed to confirm the environmental relevance of these processes. We also unambiguously identified methanol as an abundant photoproduct of lignin model compounds and as a potential precursor of other ubiquitous C 1 photoproducts, and we showed that photodegradation of lignin model compounds can be a source of OH To the authors’ knowledge, this work is the first to propose an explicit mechanism for the formation of CO from the photodegradation of terrestrial DOM. to confirm that indirect photooxidation of methanol can occur under our experimental conditions.
We further sought evidence for the formation of methanol and aromatic 3-hydroxy groups via 1H NMR spectroscopy and mass spectrometry (MS), respectively. We first established structure-reactivity relationships between photochemical reactivity and CO production. To evaluate the viability of this pathway, we investigated the photochemistry of eight lignin model compounds ( Figure 2A) focusing on specific precursors and products. (8,11) In addition, CO has been used as a proxy for the photoproduction of CO 2, CH 4, and biolabile organic carbon (5,12−16) (even though this approach proved to be inaccurate for CO 2) (5,17,18) and for developing models of mixed layer processes. (7−10) An improved understanding of OCS sources and sinks can indirectly contribute to better climate simulations, as this gas is used as a tracer of gross primary productivity. (6) Photochemical processes are significant players in the global OCS budget, but their contributions are not yet well constrained. (1,5) CO is also involved in the production of carbonyl sulfide (OCS) from DOM photolysis. Albeit contributing only a minimal fraction of the global CO budget, (1) photochemical production from DOM photolysis is relevant in remote ocean regions or in environments characterized by significant inputs of terrestrial DOM. (1−4) Among other sources, CO can be produced from the photodegradation of dissolved organic matter (DOM). ) concentrations in the troposphere, therefore influencing the residence time of greenhouse gases such as methane and halocarbons.Our results further hint that methanol may be an abundant (yet overlooked) DOM photoproduct and a likely precursor of formaldehyde, formic acid, and CO 2 and that lignin photodegradation may represent a source of hydroxyl radicals.Ĭarbon monoxide (CO) is a trace gas that plays an important role in modulating hydroxyl radical (OH This work proposes an explicit mechanism linking ubiquitous, abundant, and easily quantifiable DOM functionalities to CO photoproduction. Finally, we showed that hydroxyl radicals, likely oxidants to initiate methanol oxidation to CO, form during irradiation of all models. We then confirmed that all ArOCH 3-containing substrates undergo formal hydrolytic demethylation by detecting methanol and the corresponding phenolic transformation products. We first observed that initial CO production rates are positively correlated with initial substrate degradation rates only for models containing at least one ArOCH 3 group, regardless of other structural features. To test the reasonableness of this mechanism, we investigated the photochemistry of eight lignin model compounds. Following on this precedent, we propose that the methoxy aromatic groups of lignin act as the C source for the photochemical formation of CO from terrestrial DOM via a two-step pathway: formal hydrolytic demethylation to methanol and methanol oxidation to CO. Previous work showed that aqueous photodegradation of methoxy-substituted aromatics (ArOCH 3) produces CO considerably more efficiently than aromatic carbonyls. Carbon monoxide (CO) is the second most abundant identified product of dissolved organic matter (DOM) photodegradation after CO 2, but its formation mechanism remains unknown.
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